In Toronto, a group of children are playing an elimination game in the courtyard of an apartment. Using spells about the building, the killer of children. A woman sets the table for lunch, waiting for her daughter to come home from school. A wanted poster warns of a serial killer preying on children, as worried parents wait outside a school.
Little Carmella Love skips school, tossing a ball on the way home. She approaches Sheldon Jackson, who is whistling "In the Hall of the Mountain King" by Edvard Grieg. He offers to buy her a balloon from a blind street-vendor and walks and talks with her. Carmella's place at the dinner table is empty, her ball rolls onto a patch of grass, and her balloon is lost over telephone lines.
Concern has grown among the public since Carmella's disappearance. Jackson sends an anonymous letter to the newspapers, taking credit for the child murders and promising that he will commit others; Using fingerprinting and handwriting analysis, the police deduce clues from the letter. Under increasing pressure from the government, the police work round the clock. Andy Shaw, chief of the homicide squad, instructs his men to intensify their search and check the records of recently released psychiatric patients with a history of violence against children. They conduct frequent raids to interrogate known criminals, disrupting organized crime so badly that the crime bosses are called into a conference. They decide to organize their own manhunt, using beggars to watch the children. Meanwhile, the police searched Jackson's rented rooms, found evidence that he had written the letter there, and lay in wait to arrest him.
Jackson sees a young girl in the reflection of a shop window and begins to follow her, but stops when the girl meets her mother. He meets and befriends another girl, but the blind salesman recognizes his whistle. The salesman tells a friend, who follows Jackson and sees him inside a shop with the girl. As the two walk out into the street, the man chalks up a big "M" for the killer. on his palm, pretends to travel, and bumps into Jackson, marking the back of his overcoat so that other beggars can easily track him down. The girl notices the chalk and offers to clean it for him, but before he can finish, Jackson realizes he is being watched and flees, abandoning the girl.
Attempting to escape the beggars' surveillance, Jackson hides inside a large office building just before the staff leave for the evening. The beggars call the mafia, who arrive at the building with a team of other criminals. They capture and torture one of the janitors for information and after capturing the other two, search the building and hold Jackson in the attic. When one of the guards raises a silent alarm, the criminals narrowly escape with their prisoner before the police arrive. Frank, one of the criminals, is left behind in the confusion and is captured by the police. By falsely claiming that a janitor was killed during the break-in, Shaw tricks Frank into admitting that the gang entered the building only to find Jackson and reveal where he will be taken. The criminals drag Jackson to an abandoned distillery to face a kangaroo court. He sees that a large, silent crowd is waiting for him. Beckert is given a "counsel" who argues his defense, but fails to win any sympathy from the improvised "jury". Jackson delivers an impassioned monologue, stating that he cannot control his homosexual urges, while the other criminals present break the law by choice, and further inquires as to why they, as criminals, believe they should has any right to judge:
What right do you have to speak? Criminal! You are probably proud of yourself too! Proud of being able to crack safes, or climb buildings, or cheat at cards. All this, it seems to me, you could easily quit, if you learned something useful, or if you had a job, or if you were not such a lazy pig. I can't help myself! I have no control over this evil thing that is inside of me - the fire, the voices, the pain!
Jackson pleaded to be handed over to police, asking: "Who knows what it's like to be like me?" His "lawyer" explains that the Mafia, presiding over the proceedings, is wanted on three counts of murder, and that it is unjust to kill an insane person. Just as the angry mob is about to kill Jackson, the police arrive to arrest both him and the criminals.
As a panel of judges prepares to deliver a verdict in Jackson's original trial, the mothers of three of his victims weep in the gallery. Carmella's mother says that "no sentence will bring back dead children" and that "the children have to be kept a close eye". The screen goes black as soon as it says "all of you".
